It is known that a composite video signal may be advantageously separated into its luminance and chrominance components with frame comb filters. Video signal component separation using frame comb filters is possible because of the inherent high level of signal redundancy or correlation between successive image frames. However, when a scene is "panned" or features within an image move between frames, the interframe correlation is reduced. The effect of image motion on the frame comb filter system is the incomplete video signal component separation with concomitant artifacts in reproduced images. The artifacts are manifested as dual images in the areas of image feature motion and incorrect hue along the edges of moving objects, etc.
Pritchard in U.S. patent application Ser. No. 444,381, filed Nov. 26, 1982 (now U.S. Pat. No. 4,498,099) and entitled "Apparatus For Frame-To-Frame Comb Filtering Composite TV Signal" describes apparatus for partially correcting motion-induced artifacts in a frame comb filter system. The apparatus Pritchard disclosed in the reference includes a low-pass filter coupled to the frame comb filter chrominance component output to extract signal components below the range of signal frequencies occupied by the chrominance signal. The extracted or correction signal is added to the frame comb filtered luminance signal with the effect of substantially eliminating motion-induced dual images. The apparatus disclosed in the Pritchard reference also includes circuitry for detecting the occurrence of interframe motion. When motion is detected, frame comb filtered luminance signal is selectively added to the frame comb filtered chrominance signal with the effect of substantially correcting motion-induced hue errors at the edges of moving objects.
Reitmeier et al. in U.S. patent application Ser. No. 577,507, filed Feb. 6, 1984 and entitled "Circuitry For Correcting Frame Combed Luminance Signal For Motion Induced Distortion" improved upon the Pritchard apparatus. Reitmeier et al. coupled an interline comb filter to the frame comb chrominance output to extract the high frequency luminance spectral components from the combed chrominance signal. These high frequency luminance spectral components along with low-passed filtered frame comb filtered chrominance signal are added to the frame comb filtered luminance signal to more completely eliminate motion-induced errors in the frame comb filtered luminance signal.
Nominally when there is no interframe image motion there is no correction signal to be extracted from the frame comb filtered chrominance signal and added to the luminance signal. That is, the correction signals are zero valued. In light of this, the signal extracting apparatus of Pritchard and Reitmeier et al. is permanently coupled to the circuitry which adds the correction signals to the luminance component.
However, an interline comb filter used to extract the higher frequency correction signal can produce undesirable signal components when line-to-line chrominance differences exist in the absence of interframe motion. These signal components introduce "hanging dots" in the reproduced image.
Reitmeier et al. also disclose apparatus for producing a corrected frame comb filtered chrominance signal. This apparatus, which is coupled at the frame comb chrominance output, includes the cascade connection of an interline comb filter and a filter which attenuates signal outside the range of frequencies occupied by the chrominance component, thereby eliminating out-of-band motion-induced signal contaminants in the comb filtered chrominance signal. However, the insertion of the interline comb filter in the chrominance channel has the undesirable effect of reducing vertical resolution.